In the preparation of cephalexin with 7-ADCA being acylated with a D-phenyl glycine derivative, the recovery of the cephalexin and the working up of the reaction mixture are difficult in general. Thus in WO-A-93/12250 for instance it is described that the acylation reaction never runs to completion and the ultimate purification of the final product is hindered because the acid/base properties and solubilities of certain components (in particular 7-ADCA and phenylglycine as described in U.S. Pat. No. 4,003,896) differ little from those of the final product. As a result, coprecipitation occurs, so that impure cephalexin is obtained. In WO-A-93/12250 and U.S. Pat. No. 4,003,896 the use of a complexing agent such as naphthol is proposed. However, this entails the drawback that an additional substance alien to the process has to be added.
Another method to isolate cephalexin in pure form from a mixture containing cephalexin and minor quantities of 7-ADCA is described in JP-A-52111589. According to the method described in this Japanese publication, the mixture containing 7-ADCA and cephalexin is subjected to recrystallization in an organic solvent, for instance a mixture of dichloromethane, dimethyl sulphoxide and a lower alcohol, in the presence of an alkaline component, for instance disopropyl amine. This method, too, presents the drawback that substances alien to the process have to be added. Moreover, cephalexin losses of 18% occur in this purification method.